To experiment with digital logic circuits, you'll need a logic probe better
than the simple one you built earlier. The logic probe illustrated here is
capable of detecting both logic high and logic low states in a circuit. (If
you're really serious about experimenting with digital circuits, you should
definitely consider buying a commercial logic probe.)
To put together this more sophisticated logic probe,
you'll need these components:

Quantity

Part

Part
Number

2

1K ohm resistors

271-8023

1

Green LED

276-022

1

Red LED

276-041

3

Alligator clips

270-378

1

74LS367 IC

None

1

Solderless breadboard

276-175

You'll also need some solid copper wire
for the jumpers and some stranded copper wire for probe leads.

Here's how to put together the "better" logic probe:

1. Cut three pieces of stranded copper wire about
8 inches long and remove about 1/4 inch of insulation from each end. Attach
an alligator clip to one end of each of these wires.
2. Connect the other end of one wire to point Y1 of the solderless breadboard.
This lead is the ground wire.
3. Insert one end of another wire, the +5-volt lead, to point X1 of the
breadboard.
4. The third wire is the probe lead and is inserted into point I6 of the
board.
5. Insert the 74LS367 IC into the breadboard so that its pin 1 goes into
point F6, and its pin 9 goes into point E13.
6. Connect three jumper wires as follows: Jumper From To
1 G7
X7
2 J13
Y13
3 X5
A6

8. The red LED is mounted on the breadboard so that its cathode plugs into
point Y2 and its anode into point J2.
9. The cathode of the green LED inserts into point Y14, while its anode
plugs into point J14.

Probing

To use the logic probe, connect the ground wire and
+5-volt wire to the power supply ground and +5-volt terminals of the circuit
you're checking. Touch the probe lead to the point of the circuit you want
to check. If the green LED turns on, the point is at a logic low level, but
if the red LED lights, the point is at a logic high level. (If neither LED
lights, the point is neither at a logic high nor low state.)
When the probe is touched to a logic high point of the
circuit, current flows through the red LED, causing it to light. The probe
is also connected to the enable input pin of the 74LS367 IC. When this pin
is high, all the buffer outputs in the chip enter the high impedance state.
Since the green LED is connected to the output of a buffer, it remains off.
The green LED is effectively disconnected from the circuit since one of its
leads is connected to the buffer output, which is acting like a very large
resistor (open circuit).
When the probe is touched to a logic low point of the
circuit, no current flows through the red LED since both its leads are at
the same voltage (ground). The low signal, however, enables the 74LS367 IC
so that its buffers act normally. Since the buffer (whose output is connected
to the green LED) input is attached to +5 volts, the green LED lights up.
This happens because the output of the buffer is logic high, and as a result,
a current flows through the green LED causing it to illuminate.
Note that due to low current levels, the LEDs may be
dim and difficult to see at times.
This circuit is very handy for exploring and troubleshooting
digital circuits. It's also a worthy candidate for "permanent" mounting on
a printed circuit board, perhaps, as you'll probably use it often.